Microstructural characterization and impact toughness of a jackup rig rack steel treated by intercritical heat treatment

The novel heat treatment process for producing high toughness of a jackup rig rack steel has been developed by application of intercritical heat treatment. In the novel heat treatment process, intercritical heat treatment has been added between quenching and tempering of the conventional heat treatment process. In order to compare the novel heat treatment with the conventional one, a conventional quenching and tempering process has also been applied to the as-received TMCP rack steel. Effects of intercritical heat treatment on the microstructure and the Charpy impact behavior of the rack steel were investigated. The toughening mechanism was clarified by microstructure characterization and secondary crack propagation path analysis. Results showed that due to the intercritical heat treatment, the ductile to brittle transition temperature decreased by 44 °C. The toughness improvement comes from: (1) the microstructure of novel heat treatment steel mainly composed of upper bainite and 31.8% of ferrite and (2) the increase of high angle grain boundaries (>50°) fraction and the reduction of average effective grain size, both factors provide efficient resistance to fracture.